There is now considerable interest in the processing of vegetable and/or animal biomass to obtain fuels such as biodiesel and ethanol, in place of non-renewable fossil fuels, and thus contribute to improvement of the conditions of the environment.
Starting from vegetable and/or animal biomass, triglyceride-rich organic oils are usually obtained, and are widely used in industry.
The hydroconversion of these triglyceride-rich organic oils, mixed with diesel oil from petroleum, is an advantageous alternative that adds quality to the fuel produced, as described in patent document PI 0500591-4 for a process for conversion to paraffinic hydrocarbons with boiling point in the diesel oil range.
In general, a process of hydroconversion of organic oils, obtained from vegetable and/or animal biomass, comprises the reaction of hydrogen with the fatty acids making up the molecules of the triglycerides to produce paraffinic hydrocarbons.
The process of hydroconversion, designated HDT, includes the passage of a hydrocarbon stream, in contact with a hydrogen stream, in a fixed catalyst bed, under conditions of pressure between 1 MPa and 15 MPa, and temperature between 280° C. and 400° C. Commercial catalysts are usually employed in the form of metal oxide (Ni and Mo, Co and Mo, Ni and W), supported on gamma alumina, said catalysts being sulphided to obtain the most active form of the catalyst bed in the process.
An important factor in the conventional HDT process, making it difficult to use organic oil feeds obtained from biomass, is the highly exothermic character of the reactions of hydroconversion of the triglycerides of the feed in a reactor that operates adiabatically with increasing temperature along the catalyst bed. However, to minimize the undesirable effects of excessive temperature along the catalyst bed, an HDT reactor can be designed with more than one catalyst bed, with injection of a recycle stream between the beds, to reduce the rate of temperature rise in the catalyst bed of the reactor.
To summarize, a process of HDT of feed containing triglycerides is based on the molecular structure of the constituents of the feed and on the characteristics of the catalyst employed in the process. In conditions of hydroconversion, initially there are reactions of hydrogenation of the double bonds, followed by reactions of thermal cracking of the saturated molecules of higher molecular weight. The size of the saturated molecules promotes the reactions of thermal cracking, in conditions of high temperatures, forming carboxylic acids and acrolein. For example, in the conversion of soya oil, the carboxylic acids may undergo thermal degradation by reactions of decarboxylation resulting in nC17 and CO2, as well as reactions of decarbonylation with the production of nC17, CO and water; and reactions of dehydration producing nC18 and water. It is also thought that a molecule of acrolein reacts in the presence of the catalyst and hydrogen producing propane, and the CO also reacts, producing methane and water.
Also applied for production of fuels, the processes of catalytic cracking of organic oils obtained from vegetable and/or animal biomass represent an alternative to the processing of petroleum distillates, and are also applied for the production of light and aromatic olefins, which are of considerable economic value for the petrochemical industry.
Usually, in petrochemical fluid catalytic cracking—FCC, feeds are processed with boiling points from the range of naphthas up to that of atmospheric residues, with the aim of producing hydrocarbons of even smaller molecular size than those found in a gasoline, in particular molecules of the olefins ethylene and propylene (C2= and C3=). Usually, these products are maximized by increasing the conversion, with a decrease in the production of heavy products such as decanted oil (DO) and light cycle oil (LCO), as well as through selectivity, minimizing the formation of undesirable by-products such as coke and fuel gas.
To achieve this aim, the catalytic system is modified, normally by adding, to a typical catalyst, a special component that is able to convert olefins with five to eight carbon atoms to smaller olefins.
Optionally, an increase in reaction temperature is required, to a value that may exceed 620° C. at reactor outlet (TRX). However, high reaction temperatures impair the selectivity of the reactions of catalytic cracking, producing an undesirable increase in yield of fuel gas. High temperatures also promote the formation of aromatic hydrocarbons with boiling points in the range of gasoline and of LCO, which are poorly reactive products in catalytic cracking and interrupt the sequence of reactions that would produce desirable light products. Another negative aspect of high temperature is the production of butadienes, coke precursors which may be deposited in the transfer line and in the reactor vessel.
Besides the reaction temperature and the specificity of the catalyst, another important aspect for the reactions of cracking in a petrochemical FCC process is the initial contact of the catalyst with the feed, which has a decisive influence on the conversion and the selectivity of the process for generating higher-value products. Therefore it is important to have the maximum possible atomization of the feed in the injectors, in order to guarantee the homogeneity of the catalyst/feed mixture, at the inlet of the FCC reactor.
For production of olefins in the petrochemical FCC, catalysts can be used that contain zeolites of type Y, supported on an active matrix of alumina and a binder, said zeolites preferably having a low content of rare earths in their composition. It is also possible to use zeolites of type ZSM-51, a special component capable of converting olefins with five to eight carbon atoms to smaller olefins.
The catalysts used for petrochemical cracking should contain zeolites of type USY, REY and ZSM, the zeolites of type ZSM with silica-alumina ratio equal to 10 or more, including for example zeolites ZSM-5 (MFI), ZSM-11, ZSM-12 and ZSM-35, being the main ones used for the conversion of hydrocarbons.
In the petrochemical FCC of vegetable oils, we may mention in particular patent PI 8304794, which teaches the process conditions for obtaining greater conversion of a feed of soya oil, compared with the results for FCC of a usual feed of gas oil.
U.S. Pat. No. 7,288,685 teaches a petrochemical FCC process for the production of olefins from a feed of vegetable oils, previously purified of metallic contaminants, using a zeolitic catalyst, containing zeolites of type ZSM as principal constituent. In general, the results from a petrochemical FCC process show an increase in the production of olefins accompanying the increase in severity of the process, but there is also increased formation of fuel gas and coke, as well as other hydrocarbons heavier than the constituents with boiling point in the naphtha range.
Moreover, petrochemical FCC can be applied in combined processing of biomass, as taught in U.S. Pat. No. 5,504,259 for the production of gasoline from biomass and plastic waste such as PVC and polyethylene. In this case, the feed first undergoes a pyrolysis stage and the oily liquid product from pyrolysis is then submitted to catalytic cracking, to generate light olefins that are oligomerized with alcohol for the production of ether, which can be added to gasoline to increase the overall yield of the product.
Therefore there is clearly a search for alternatives for the processing of biomass to obtain products of higher economic value, such as light olefins (C2= and C3=) and gasoline (C5+).
In the following, a process is described that combines hydroconversion and catalytic cracking so as to obtain gains in conversion and selectivity for the production of light olefins (C2= and C3=), from a feed containing triglycerides of vegetable and/or animal biomass, with a high degree of conversion of the process.